1. Field of the Invention
The present invention relates to a radar apparatus provided with a reception array antenna consisting of a plurality of antenna elements and constructed to detect a bearing of a target by signal processing on individual element signals received through the respective antenna elements.
2. Related Background Art
A bearing of a target can be determined by a scan with an antenna beam narrowed down to a desired width.
Scanning methods can generally be classified into mechanical scanning methods and electronic scanning methods and a digital beamforming (DBF) scanning method can be listed as one of the electronic scanning methods.
The DBF scanning method is a method of carrying out the scan with the antenna beam by making use of the DBF technology in which the antenna beam can be formed in any desired direction by using the array antenna with a plurality of antenna elements as a receiving antenna, performing a phase shift process by digital signal processing on the individual element signals received through the respective antenna elements, and carrying out a synthesis process.
The DBF scanning method eliminates the need for rotation of the antenna, which was required in the mechanical scanning methods. Accordingly, it obviates the need for a driving mechanism for rotating the antenna and thus has the advantages of being resistant to vibration and permitting reduction in size and weight. While making the most of such advantages, research has been conducted heretofore on application to on-vehicle radar apparatus.
Incidentally, for enhancing the bearing resolution in the DBF scanning method, it is necessary to increase the size of the antenna and increase is inevitable in the digital operation load for formation of beam. For that reason, the bearing resolution has a limit under the constraints that the radar apparatus has to be installed in a limited space or that the detection result has to be given within an effective time.
Particularly, the bearing resolution degrades with distance to the target. For example, supposing two preceding cars running with a predetermined spacing between them are targets to be detected, it will become more difficult to separate and detect the two cars as the distance increases.
There were thus needs for the radar apparatus capable of enhancing the bearing resolution as occasion demands.
A radar apparatus according to the present invention has been accomplished in order to meet such needs and comprises a reception array antenna having a plurality of antenna elements, first bearing detecting means for detecting a bearing of a target by carrying out signal processing on individual element signals received on element-by-element basis through the respective antenna elements, and second bearing detecting means for detecting a bearing of a target by carrying out signal processing different from that of the first bearing detecting means, on the individual element signals received through the respective antenna elements.
Since the apparatus comprises two bearing detecting means of the first bearing detecting means and second bearing detecting means as target bearing acquiring means, both results of the detection by the two means, or either one of results of the detection can be selectively utilized as occasion demands.
It is desirable that the first bearing detecting means impose a lighter operational load for detection of the target bearing than the second bearing detecting means and that the second bearing detecting means posses a higher target bearing resolution than the first bearing detecting means. The first bearing detecting means is applied to detection of a target existing in a relatively near range and the second bearing detecting means to detection of a target existing in a relatively far range.
This radar apparatus is desirably an FM-CW radar apparatus which uses a transmitted signal of frequency-modulated continuous wave and which acquires the distance to the target from a beat frequency between a received signal through the reception array antenna and the transmitted signal.
Further, it is desirable that the apparatus further comprise a reception circuit for acquiring beat signals by mixing the transmitted signal with the received signals through the antenna elements and converting the beat signal gained every antenna element to a digital beat signal, that the first bearing detecting means be means for performing a digital beamforming process on a plurality of element-by-element digital beat signals to form an antenna beam in a plurality of directions and detecting the bearing of the target, and that the second bearing detecting means be means for carrying out an analysis by digital signal processing on a phase difference between the plurality of element-by-element digital beat signals to detect the bearing of the target as point information.
The first bearing detecting means has the advantages of being capable of detecting not only the center position of the target but also the width thereof and necessitating the processing time shorter than the second bearing detecting means, and the second bearing detecting means has the advantage of the bearing resolution higher than the first bearing detecting means. The target can be detected with accuracy by properly selecting a bearing detecting means to be used in consideration of these advantages.
One of criteria for selection of the first and second bearing detecting means can be the magnitude of the beat frequency. For example, the target bearing can be detected using the first bearing detecting means for the beat signal of the beat frequency not more than a predetermined value, and the target bearing can be detected using the second bearing detecting means for the beat signal of the beat frequency larger than the predetermined value.
The beat frequency is proportional to xe2x80x9cdistancexc2x1relative speed.xe2x80x9d Namely, it is roughly proportional to the distance though there occurs some error depending upon the difference in relative speed. When the target is present in the near range, the beat frequency takes a smaller value than when it is present in the far range.
On the other hand, the bearing resolution of the first bearing detecting means degrades with distance to the target. As the distance to the target increases, it is often the case that it becomes unnecessary to dare to acquire the information about the width of the target.
Thus, the first bearing detecting means is used for the beat signal of the beat frequency not more than the predetermined value, while the second bearing detecting means for the beat signal of the beat frequency larger than the predetermined value, whereby it becomes feasible to acquire the width information for the target in the near range as well while maintaining the necessary bearing resolution throughout the entire range of distances to be detected.
This radar apparatus is desirably provided with beat frequency detecting means for detecting a beat frequency indicating an approximate distance at which a target is assumed to exist, prior to the detection of the bearing of the target by the first bearing detecting means or by the second bearing detecting means.
When the target bearing detection by the first bearing detecting means or by the second bearing detecting means is carried out with limiting the detection to the beat frequency gained by the beat frequency detecting means, the processing time can be reduced greatly, as compared with the case wherein the target bearing detection is carried out throughout the entire beat frequency range.
The first bearing detecting means can detect the bearing of the target from a position of a peak in a power distribution with a variable of bearing and detect the target width from a peak width, i.e., from a width of the distribution at a position where power is lowered by a predetermined amount from the peak position.
If the target width is not less than a predetermined value, there is the possibility of fusion of targets. In that case, the bearing of the target is again detected at the same beat frequency by the second detecting means, whereby the targets can be separately detected.
The second bearing detecting means can be means for detecting the target bearing by an analytical operation using an adaptive array antenna filter (hereinafter referred to as AAAF).
The processing time can be shortened by setting initial values of weights in the analytical operation with AAAF, based on the result of recognition by the first bearing detecting means or based on the result of recognition of target in previous detection.
A radar apparatus according to another aspect of the present invention comprises a reception circuit for mixing a transmitted signal with the received signals through the plurality of antenna elements to acquire beat signals and converting the beat signal gained every antenna element to a digital beat signal, and beat frequency detecting means for detecting a beat frequency indicating an approximate distance at which a target is assumed to exist, each of the first and second bearing detecting means is means for performing a digital beamforming process on a predetermined frequency component of the plurality of element-by-element digital beat signals to form an antenna beam in a plurality of directions and thereby detecting the bearing of the target, the first bearing detecting means performs the digital beamforming process for a frequency detected by the beat frequency detecting means and being not more than a predetermined frequency or for a frequency near said frequency, and the second bearing detecting means performs the digital beamforming process for all frequencies from the predetermined frequency to a maximum frequency of a detection range.
A peak level in a beat frequency spectrum before the DBF synthesis process becomes lower with distance to the target, or with increase in the value of the beat frequency. For that reason, it is possible to fail to detect the peak frequency for a far target.
On the other hand, when the beat frequency spectrum obtained with the antenna beam formed by the DBF synthesis process is compared with that before the DBF synthesis process, the former has a sharper peak rise due to the target when the target is present in the direction of the antenna beam.
Since the DBF synthesis process is carried out for the beat frequencies larger than the predetermined frequency, throughout the entire frequency range up to the maximum frequency of the detection range, according to the present invention, it is feasible to detect a peak of the beat frequency spectrum due to the target, which could be missed in the beat frequency spectrum before the DBF synthesis process.
In addition, for the target in the relatively near range, the peak frequency is detected from the beat frequency spectrum before the DBF synthesis process and the detection of the distance and direction of the target by the DBF synthesis process is carried out at the frequency or at a frequency near it, whereby the operational load becomes smaller than when the DBF synthesis process is carried out throughout the entire frequency range.
The FM-CW radar apparatus of the present invention desirably comprises lane shape acquiring means for acquiring a shape of a lane on which a vehicle is running, when mounted on the vehicle, and is arranged so that when it is determined that a target detected by the first or second bearing detecting means is present on the lane acquired by the lane shape acquiring means, the DBF synthesis process at beat frequencies greater than the beat frequency corresponding to the target is suspended before a new beat frequency is acquired.
When the FM-CW radar apparatus is mounted on a vehicle and used for detecting the behavior of a preceding car running on the same lane, it becomes less necessary to detect vehicles ahead of the preceding car once the preceding car running on the same lane is detected.
In this FM-CW radar apparatus, when the target on the same lane is detected, the detection of the target ahead thereof is suspended, so as to greatly decrease the operational load for the DBF synthesis process.
The radar apparatus according to still another aspect of the present invention comprises a reception circuit for mixing the transmitted signal with the received signals through the plurality of antenna elements to acquire beat signals and converting the beat signal gained every antenna element to a digital beat signal, the first and second bearing detecting means are means for performing the digital beamforming process on the plurality of element-by-element digital beat signals to form an antenna beam in a plurality of directions and thereby detecting the bearing of the target, and the number of antenna beams formed per unit angle in the first bearing detecting means is smaller than that in the second bearing detecting means.
The forming number of antenna beams per unit angle corresponds to a scanning angle resolution or a scan pitch angle. Thus the scanning angle resolution becomes higher while the scan pitch angle becomes smaller with increase in the forming number of antenna beams per unit angle. Namely, in this radar apparatus, the second bearing detecting means has a higher scanning angle resolution and, in other words, a smaller scan pitch angle than the first bearing detecting means.
When the scan pitch angle is predetermined and thus the scanning angle resolution is constant, the length resolution along the scanning direction varies according to the distance to the target. For example, the angle subtended by the target of width A becomes smaller as the distance to the target increases. Therefore, if the scanning angle resolution is constant, the length resolution in the width direction of the target will become lower with distance to the target.
For that reason, if the scanning angle resolution is determined on the basis of the near targets, the length resolution in the scanning direction will become insufficient for the far targets. On the other hand, if the scanning angle resolution is determined on the basis of the far targets, the length resolution in the scanning direction for the near targets will be higher than necessary. The resolution higher than necessary is nothing but execution of the DBF synthesis process toward too many directions, so as to increase the processing time.
However, this radar apparatus solved the above problem by applying the bearing detection by the first bearing detecting means to the near targets and applying the bearing detection by the second bearing detecting means to the far targets.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.